1. Field of the Invention
The present invention relates to a multilayer thin-film structure for a magnetoresistive device employed in magnetic heads, position sensors, rotation sensors and the like.
2. Description of the Related Art
An NiFe alloy (Permalloy) is a known magnetoresistive (MR) material which has been used for forming multilayer thin-film structures for MR devices. Generally, the percentage change in resistance of a Permalloy thin film is in the range of 2 to 3%. Accordingly, magnetoresistive thin films having magnetoresistive ratios (MR ratios) greater than that of Permalloy have been desired to cope with increase in linear density and track density in magnetic recording or increase in the resolving power of magnetic sensors.
Recently, a phenomenon called giant magnetoresistive effect has been found in a multilayer thin-film structure, such as a multilayer thin-film structure consisting of alternate layers of Fe thin films and Cr thin films or alternate layers of Co thin films and Cu thin films. In such a multilayer thin-film structure, the magnetizations of the ferromagnetic layers of Fe or Co interact magnetically through the nonmagnetic layers of Cr or Cu and the magnetizations of the stacked ferromagnetic layers are coupled to maintain an antiparallel alignment; that is, in this multilayer thin-film structure, the direction of magnetizations of the ferromagnetic layers spaced alternately with the nonmagnetic layers are opposite to each other. When an appropriate external magnetic field is applied to such a structure, the magnetization directions of the ferromagnetic layers are aligned in a direction.
In this multilayer thin-film structure, it is known that the state where the magnetizations of the ferromagnetic layers are in an antiparallel alignment and the state where the magnetizations of the ferromagnetic layers are in a parallel alignment differ from each other in the scattering mode of conduction electrons in the interface between the ferromagnetic Fe layers and the nonmagnetic Cr layers or between the ferromagnetic Co layers and the nonmagnetic Cu layers. Consequently, the electric resistance is high when the magnetization directions of the ferromagnetic layers are in an antiparallel alignment, the electric resistance is low when the magnetization directions of the ferromagnetic layers are in a parallel alignment, which produces the so-called giant magnetoresistive effect causing a resistance change at a high percentage ratio greater than that of resistance change in a Permalloy film. Thus, these multilayer thin-film structures have an MR producing mechanism basically different from that of the conventional single NiFe film.
However, since the magnetic interaction between the ferromagnetic layers of those multilayer thin-film structures that acts in an effort to set the magnetizations of the ferromagnetic layers in an antiparallel alignment is excessively strong, a very intense external magnetic field must be applied to those multilayer thin-film structures to set the magnetization directions of the ferromagnetic layers in a parallel alignment. Therefore, a large resistance change cannot be expected unless a very intense magnetic field is applied to the multilayer thin-film structures, and hence magnetic heads that detect an applied magnetic field of a very low intensity created by a magnetic recording medium are unable to function with satisfactorily high sensitivity when such a multilayer thin-film structure is incorporated into those magnetic heads.
It may be effective, for solving such problems, to determine the thickness of the nonmagnetic layers of Cr or Cu so that the magnetic interaction between the ferromagnetic layers are not excessively strong and to control the relative magnetization directions of the ferromagnetic layers by another means other than the magnetic interaction.
A technique proposed to control the relative magnetization directions of the ferromagnetic layers employs an antiferromagnetic layer, such as an FeMn layer, to fix the magnetization direction of one of the ferromagnetic layers so that the magnetization direction of the same ferromagnetic layer may not be changed by an external magnetic field, and to allow the magnetization direction of the other ferromagnetic layer to change to enable the multilayer thin-film structure to be operated by an applied magnetic field of a very low intensity.
Referring to FIG. 17 showing an MR sensor A disclosed in U.S. Pat. No. 5,159,513 employing the foregoing technique, the MR sensor A is formed by depositing a first magnetic layer 2, a nonmagnetic layer 3, a second magnetic layer 4 and an antiferromagnetic layer 5 on a nonmagnetic substrate 1. The magnetization direction B of the second magnetic layer 4 is fixed by the magnetic exchange coupling effect of the antiferromagnetic layer 5, the magnetization direction C of the first magnetic layer 2 is kept perpendicular to the magnetization direction B of the second magnetic layer 4 in the absence of an applied magnetic field. Since the magnetization direction C of the first magnetic layer 2 is not fixed, the magnetization direction C can be rotated by an applied external magnetic field. When a magnetic field h is applied to the MR sensor of FIG. 17, the magnetization direction C of the first magnetic layer 2 rotates as indicated by the arrows according to the direction of the applied magnetic field h and, consequently, the first magnetic layer 2 and the second magnetic layer 4 become different from each other in magnetization rotation causing resistance change that enable the detection of the applied magnetic field.
Another MR sensor B shown in FIG. 18 has one magnetic layer having a fixed magnetization direction and one magnetic layer having a free magnetization direction. As shown in FIG. 18, the MR sensor B is formed by sequentially depositing an antiferromagnetic layer 7 of NiO, a magnetic layer 8 of an NiFe alloy, a nonmagnetic layer 9 of Cu, a magnetic layer 10 of an NiFe, a nonmagnetic layer 11 of Cu, a magnetic layer 12 of an NiFe alloy and an antiferromagnetic layer 13 of an FeMn alloy in that order on a substrate 6. The antiferromagnetic layers 7 and 13 fix the magnetization directions of the adjacent ferromagnetic layers 8 and 12, and the magnetization direction of the ferromagnetic layer 10 sandwiched between the nonmagnetic layers 9 and 11 and disposed between the ferromagnetic layers 8 and 12 rotates according to the direction of an applied external magnetic field.
In the MR sensor of FIG. 17 (FIG. 18), the resistance of the first magnetic layer 2 (the magnetic layer 10) varies linearly in a satisfactory linearity with the variation of the applied magnetic field of a very low intensity and hence the MR sensor is able to measure the applied magnetic field of intensity varying in a wide range. When the first magnetic layer 2 is formed of a soft magnetic material, such as an NiFe alloy, the highly permeable axis of hard direction of magnetization of the soft magnetic material can be used as the magnetization direction and the degree of hysteresis is small.
However, since the MR sensor of FIG. 17 or the MR sensor of FIG. 18 fix the magnetization direction of the second magnetic layer 4 adjacent to the antiferromagnetic layer 5 of FeMn or the magnetization directions of the ferromagnetic layers 8 and 12 by the antiferromagnetic layer 7 of NiO underlying the ferromagnetic layer 8 and the antiferromagnetic layer 13 of an NiFe alloy overlying the ferromagnetic layer 12 and keeps the magnetization direction of the intermediate magnetic layer 10 free, the number of the interfaces between the NiFe alloy layer (magnetic layer) and the Cu layer (nonmagnetic layer), which contributes to giant magnetoresistive effect, cannot be increased, which is a significant restriction on the magnitude of the MR ratio. Accordingly, the MR sensors of the structures shown in FIGS. 17 and 18 are utterly unable to achieve MR ratios on the order of 10 to 20%. The material forming the antiferromagnetic layers 5 and 7, i.e., FeMn, is not advantageous in corrosion resistance and environment resistance.
Furthermore, when forming the layers respectively having spontaneous magnetization directions, i.e., the directions of axes of easy magnetization of magnetic anisotropy, oriented at an angle of 90.degree. with respect to each other in a vacuum chamber in fabricating the MR sensor shown in FIG. 17 or 18, the applied magnetic field must be turned for each layer, which requires a film forming apparatus of a complex construction capable of turning the applied magnetic field and hence increases equipment costs.
Another previously proposed structure capable of controlling relative magnetization direction is formed by alternately depositing a plurality of ferromagnetic layers in which the reversion of magnetization by an external magnetic field is difficult, such as Co layers, and a plurality of ferromagnetic layers having a small coercive force and a soft magnetism, such as an NiFe alloy layers, with nonmagnetic layers, such as Cu layers, interposed therebetween.
FIG. 19 shows an MR device D developed by the application of such a technique, mentioned in Journal of the Magnetics Society of Japan, Vol. 15, No. 2, pp. 431-436 (1991). This MR device D is formed by stacking, on a substrate 15, a plurality of layered structures each formed by sequentially depositing a nonmagnetic layer 16 of Cu, a magnetic layer 17 of an NiFe alloy having a low coercive force, a nonmagnetic layer 18 of Cu, a magnetic layer 19 of Co having a high coercive force, a nonmagnetic layer 20 of Cu, a magnetic layer 21 of an NiFe alloy of a low coercive force, a nonmagnetic layer 22 of Cu and a magnetic layer 23 of Co of a high coercive force.
In the MR device of FIG. 19, since the magnetization directions of the magnetic layers 19 and 23 having a high coercive force are difficult to change by an external magnetic field, and the magnetization directions of the magnetic layers 17 and 21 having a low coercive force are easy to reverse, it is possible to change over from one of an antiferromagnetic state, i.e., a state of antiparallel magnetization where the resistivity is large, and a ferromagnetic state, i.e., a state of parallel magnetization where the resistivity is small, to the other by an applied magnetic field of a low intensity. Therefore, the number of the magnetic layers can be increased and layers of materials having problems in environment resistance, such as FeMn, need not be used. However, the MR device of FIG. 19 has the following problems.
An NiFe alloy forming the magnetic layers of a low coercive force and Co forming the magnetic layers of a high coercive force are utterly dissimilar substances, conduction electrons receive different potentials from those magnetic layers and spin-dependent electron scattering that contribute to giant magnetoresistive effect increases in the interfaces. Therefore the MR ratio cannot be increased very much.
Since the magnetocrystalline anisotropy of Co forming the high-permeability magnetic layers is large, it is difficult to control induced magnetic anisotropy when forming the Co magnetic layers in a magnetic field and to form the Co magnetic layers in uniform uniaxial magnetic anisotropy, and the complete design of a layered structure having layers having axes of spontaneous magnetization oriented at an angle of 90.degree. with respect to each other like the structure shown in FIG. 17 or 18; that is, it is difficult to magnetize the layers having magnetization directions oriented at an angle of 90.degree. with respect to each other by the control of magnetization using the difference in coercive force between the magnetic layer of a high coercive force and the magnetic layer of a low coercive force. Accordingly, the upper limit of the MR ratio of the MR device of FIG. 19 is on the order of 10%.